1. Field of the Invention
The present invention relates to the detection of gas traces. It more specifically relates to the cavity ring down spectroscopy (CRDS) method.
2. Description of the Related Art
The major principles of the CRDS method will be reminded in relation with FIG. 1. The method consists of emitting light from a laser 1 in a resonant optical cavity 2. An optical isolation system or a deflector 3 is used to avoid feedback effects of the resonant cavity on the laser. The light coming out of the resonant cavity is received by a photodetector 4 and is sent to an analyzer 5. Assuming that photons have been injected into cavity 2, once the injection of photons into the cavity by the laser is interrupted, for example, by cutting off the laser or by reflecting its beam, the photons remain trapped in the cavity and decrease exponentially along time. If the cavity is empty, or for a wavelength that does not correspond to an absorption line of a gas contained in the cavity, this decrease will exhibit a certain time constant essentially determined by the mirror losses at the considered wavelength. If the cavity contains a chemical species having an absorption line at the wavelength of the injected photons, this time constant will be reduced.
This is illustrated in FIG. 2, which shows the intensity collected by photodetector 4 as a function of time. It is assumed that at time T0, the excitation is interrupted and that there exists a given density of photons I0 in the cavity. If the cavity contains no absorbing species at the considered frequency, the fall time has a first value t1. If the cavity contains absorbing species, the fall time becomes t2. The concentration of absorbing species in the cavity is related to the difference t2xe2x88x92t1.
Many studies and laboratory experiments have been carried out to use and improve this gaseous species detection method. It has quickly been understood that, to turn laboratory experiments into a method likely to be implemented by an industrial device at low cost, a continuous laser had to be used.
The first experiments on the CRDS method have been carried out with pulsed lasers providing very intense power pulses with a relatively wide spectrum. A sufficient number of photons could then be injected into the cavity to perform measurements. However, this method would come up against the major disadvantage of the complexity and cost of pulsed lasers. On the other hand, it has long since been suggested (see D. Z. Anderson et al., Applied Optics, Volume 23, 1984, p. 1238-1245) to use a continuous laser as a source. All these known techniques are discussed in detail in U.S. Pat. No. 5,528,040 of K. K. Lehmann filed in 1994, which also advocates the use of a continuous laser as a source.
As experiments have advanced, one of the major problems to be solved has appeared to be the injection of a sufficient amount of light into the resonant cavity.
Another problem that is posed by prior art devices is the fact that they are generally complex since they include control systems to set the laser frequency at the time of interruption of the laser/cavity coupling.
Thus, an object of the present invention is to provide a method for measuring traces of a chemical species by the use of a resonant cavity spectroscopy method in which the injection of photons into the resonant cavity from a continuous laser is optimized.
Another object of the present invention is to provide a method that is easy to implement due to its providing no frequency control of the system.
To achieve these objects, the present invention provides a method of gas trace detection by a laser coupled to a resonant optical cavity containing a chemical species to be analyzed, including the steps of providing that the coupling between the laser and the cavity is such that the light is only sent back to the laser when the cavity is in a resonance mode and at the resonance frequency; providing a semiconductor laser of a type adapted to providing an emission amplified and thinned down at the reinjected frequency, and such that, when a current rectangular pulse is applied thereto, its frequency moves from a determined initial frequency to a determined final frequency; exciting the laser by a first current rectangular pulse so that the laser frequency sequentially locks on successive modes of the cavity; measuring the fall time of the light intensity in the cavity at the end of said rectangular pulse; and repeating the steps of excitation and measurement for successive current rectangular pulses, to cover a spectral range to be analyzed.
According to an embodiment of the present invention, the laser is a laser diode.
According to an embodiment of the present invention, the laser is excited by sequential current rectangular pulses of increasing intensity.
According to an embodiment of the present invention, the laser is excited by sequential current rectangular pulses of increasing length.
According to an embodiment of the present invention, the laser is excited by identical sequential current rectangular pulses, the temperature at which the laser is stabilized being incremented after each rectangular pulse.
According to an embodiment of the present invention, the cavity is of V-shaped type, comprised of a first oblique mirror with respect to the direction of incidence of the laser, a second mirror orthogonal to the direction of incidence of the laser, and a third mirror forming a cavity with the first two mirrors.
According to an embodiment of the present invention, the cavity is a conventional cavity with two mirrors and a polarizing isolator is arranged between the laser and the cavity to prevent the returning to the laser of a direct reflection on the rear surface of the entrance mirror and to transmit to the laser a radiation having undergone a resonance in the cavity.
According to an embodiment of the present invention, the cavity is set to operate in a mode close to a degenerated mode, the secondary transverse modes being all gathered on a same side of a main corresponding transverse mode, the laser performing a scanning in the direction starting from the side opposite to that where the secondary lateral modes are found.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.